This invention relates to improved methods and apparatus for injection molding of articles, such as rubber or plastic articles, having a mottled appearance.
One injection molding technique which has been heretofore employed involves the use of an injector unit of the type having a barrel and a plunger disposed for reciprocating movement within an inner bore of the barrel. The inner bore-defining surface of the barrel is heated, usually by means of heating bands which encircle the outer periphery of the barrel. By inserting molding materials of common or diverse coloration ahead of the plunger and then advancing the plunger, the materials are displaced toward the discharge orifice of the barrel. Contact with the heated surface of the bore causes the materials to become heated and plasticized. Material-spreading means might be employed in the barrel to increase the rate of heat exchange occurring between the material being displaced along the bore and the heated bore surfaces.
More recently, injection molding techniques have been somewhat revolutionized by the introduction of screw-type plasticating units.
The basic screw injector apparatus includes an elongated barrel and a screw which extends longitudinally through the barrel. The screw has a helical land or thread on its surface which cooperates with the internal surface of the barrel bore. The screw is rotated about its own axis to work the molding material and feed it toward the outlet end of the barrel. The working of material generates heat causing the material to melt as it progresses along the screw. Characteristic of screw-type injectors is an intense mixing of the materials which occurs during feeding, assuring that, upon reaching the end of the screw, the melt will be of substantially uniform temperature and consistency, and free of small gels and other fine-structure agglomerations.
Due to the superior performance of screw-type injectors over those previously utilized, especially involving such features as rate of plasticization, efficiency of heat transfer, degree of mixing, and overall melt quality, current high-production molding facilities rely heavily on such screw injector units.
Although screw injectors have been found highly advantageous in providing plasticated material having substantially homogeneous properties, such material may not be acceptable for the production of certain articles. For instance, the fabrication of articles having a mottled or marbled appearance requires that materials of diverse coloration must be melted and ejected without the occurrence of undue blending or dispersion of the colors. The intense material mixing and dispersion effected by screw injectors as conventionally constructed and utilized, would obviate the formation of such a color pattern. Hence, until now, the versatility of screw injectors has been somewhat limited.
It is, therefore, a general object of the invention to eliminate or minimize problems of the sort previousy discussed.
It is a particular object of the invention to provide novel extrusion methods and apparatus for enabling screw injectors to effectively produce articles having a mottled appearance.
It is another object of the invention to maximize the versatility and efficiency of screw injector operation.